Many applications employing optical devices are limited by optical aberrations that result from the configuration of the optical device. The term “optical aberrations” includes deviations from the correct function of the optical device that result from disruptions and distortions of optical paths associated with the device. A source of optical aberrations that is particularly limiting in the use of optical devices is the object of the optical device. The object, or sample, refers to the element that the optical device is designed to perform a particular function on. Object-induced, or sample-induced, aberrations result from the refractive properties of the object or the optical path between the object and the optical device, and may include spherical aberrations that are symmetrical around the optical axis, as well as other types of aberrations.
Aberrations can be characterized by the shift in optical path experienced by a ray of light as it travels from a point source (i.e., a point at a given depth within a sample) through the sample. For many common configurations (i.e., for a system with a set sample refractive index and a set atmospheric or objective immersion refractive index separated by an interface plane perpendicular to the optical axis) the shift in optical path is only dependent upon the depth of the point source within the sample and the angle at which the ray of light travels through the sample relative to the optical axis.
The pupil plane in an optical device is the plane where the position of each ray depends solely on the angle of emergence from the sample alone. For any given optical device, it is possible to determine where at the pupil plane a ray with a particular angle of emergence will be positioned. Accordingly, at this plane, it is possible to identify the angle of emergence from the sample for each ray based upon the position of the ray at the pupil plane. Since in common configurations the shift in optical path depends on depth and angle alone, for each depth it is possible to correct the spherical aberrations at the pupil plane by introducing an optical element that can correct each ray of light according to its optical path shift.
Spherical aberrations are particularly problematic when analyzing moderately thick objects because they are depth dependent. A detailed discussion of optical aberrations, and a method and system for correcting optical aberrations, including spherical aberrations, is provided in U.S. patent application Ser. No. 11/419,070, which is incorporated herein by reference. The '070 application describes a method and system for correcting optical aberrations in applications such as wide-field microscopy, optical tweezers and optical media read/write devices. The '070 application teaches the use of adaptive optical elements of several types, such as a liquid lens (adjusted by pressure), a deformable membrane mirror (adjusted by piezoelectric or magnetic pistons), micro electro-mechanical (MEMS) mirrors, or various liquid crystal phase and amplitude modulators (with optical properties that are controlled pixel-by-pixel electrically via the patterned surfaces holding the liquid between them). Additionally, the '070 application provides that a mirror, such as a deformable mirror, may be used as an adaptive optical element for correcting optical aberrations.
Existing deformable mirror elements that use a force at the center of the rear surface of a mirror create a parabolic shape deformation on the mirror's surface. See, e.g., U.S. Pat. No. 7,229,178, which is incorporated herein by reference. Such mirrors can be made as small as required. The resulting wavefront after reflection in such mirrors can change the focus of the impinging wave, however, they introduce an additional large component of spherical aberrations. Other types of mirrors are deformed by introducing forces applied by multiple actuators at multiple locations on the mirror (typically on a rectangular or hexagonal array of points). These mirrors have more degrees of freedom for creating arbitrary functions of distortions for shaping a wavefront reflected from their surface. However, they are problematic in that they must be sufficiently large in order to provide space for the actuators, and they are associated with high-order aberrations introduced by the array of pistons. This problem is shared by both segmented mirrors (where each mirror segment is associated with one piston for displacement and maybe additional pistons for tilt) and “shape mirrors” (where multiple pistons push on a deformable membrane mirror). Both types of mirrors create deviations from the ideally required shape at the spatial frequency of the piston array; this is called piston “print through” or “waffle.” Another kind of adaptive optical element based on liquid crystals has small size and high spatial resolution.
However, a problem shared by most available adaptive elements is their limited range of creating phase shifts (e.g., a few wavelengths for typical devices). The maximum phase shift introduced by an adaptive optical element is called “throw”. Typical adaptive devices with throws of the order of two wavelengths can correct small aberrations, but have limited capability to change the focus of optical systems. They cannot correct aberrations in moderately thick objects, and cannot change focus in a useful scale. Other types of electric and magnetic actuators that can introduce more “throw” are large and transduce a very limited amount of force. These actuators are used with soft deformable membrane mirrors and have large “print-through” effects. The large size of the mirrors necessitates long optical distances that require large and bulky optical systems.